To increase the efficiency of power amplifiers in radio frequency transmitters, the power amplifiers are frequently operated in a nonlinear region. However, such an operation method causes high-level inter-modulation distortion (IMD) to be generated, resulting in an increase in leakage power between neighboring channels. This is specifically noticeable in orthogonal frequency division multiplexing (OFDM) in which a peak-to-average power ratio (PAPR) is large. Further, since constellation (position in the complex plane of a signal) in modulation of a signal is distorted due to the nonlinearity of power amplifiers, an error vector magnitude (EVM) is deteriorated, resulting in an increase in bit error rate (BER).
Digital pre-distortion (DPD) is suggested as a method for improving the nonlinear characteristics of power amplifiers. For example, in the case of adaptive pre-distortion, which is one of the pre-distortion methods, in consideration of the nonlinearity of a power amplifier an input signal is multiplied in advance by a compensation coefficient that represents the characteristics that are the inverse of those of the nonlinearity, thereby making the input signal prior to amplification be subjected to pre-distortion processing. When the compensated input signal is amplified by the power amplifier, the characteristics (inverse characteristics described above) given to the input signal are cancelled out by the nonlinearity of the power amplifier, allowing the power amplifier to have linear output characteristics. Digital pre-distortion is a reliable method for linearization of power amplifiers in terms of efficiency and cost.